1. Field of Invention
The present invention relates to a correction method of a scanning electron microscope. In particular, the present invention relates to a correction method of a scanning electron microscope employed for dimensional measurement or the like of a highly integrated circuit semiconductor.
2. Description of the Related Art
Currently, measurement of pattern dimensions in a semiconductor device is performed by a scanning electron microscope (hereinafter, SEM). This SEM is an apparatus for observing a surface of a micro (minute) sample by scanning an electron beam on the surface of the minute sample instead of light. The SEM has a merit in that the sample can be observed in a wide range from scores of magnifications to hundreds of thousands of magnifications (the magnification in which a molecular level can be observed).
In general, in the SEM, an electron generated by an electron gun is converged to an electron beam by a group of electromagnetic lenses. This electron beam is adjusted by means of an electromagnetic lens for focal position control so as to be converged at one point on a stage. This electron beam is scanned on the stage in a two-dimensional manner by controlling a magnetic field caused by a beam-scanning electrode.
When a sample is placed on the stage, and irradiated by the electron beam, secondary electrons according to a rugged state of a sample surface are discharged from the sample surface targeted for measurement. In the SEM, the discharged secondary electrons are collected, and are detected by means of a detector such as a scintillator or a photo multiplier (photoelectric multiplier) to be electrically converted. The electrically converted electron is synchronized with a scan signal of a scanning radiation of a display section such as a CRT, whereby a secondary electron image (hereinafter, referred to as an SEM image) according to the rugged state of the sample surface is obtained. Since the scanning range of the electron beam is narrow, the stage is moved in a X-Y plane direction by means of a stage movement system, whereby the SEM image of the entire sample targeted for measurement is obtained.
In addition, the SEM enables observation of a minutely small region, and thus, it is used as an apparatus for dimensional measurement as well as sample observation. In this case, it is required to perform the correction work of dimensional measurement precision, thereby maintaining the measurement precision. Conventionally, for example, correction has been performed by obtaining how many dots on the screen the SEM image of a standard sample whose dimensions are obtained in advance correspond to.
Conventionally, it is presumed that the electron beams of the SEM are always converged at one point on the sample surface, and that an ideal irradiation position caused by an electron beam control signal coincides with an irradiation position of actual electron beam. However, it is not actually verified as to whether or not the electron beam is converged on the sample surface and as to whether or not the ideal irradiation position caused by the electron beam control signal coincides with the actual irradiation position of electron beams.
Because of this, actually, if the electron beam is not always converged at one point on the sample surface or if the ideal irradiation position of electron beam does not coincide with the irradiation position of actual electron beam, these cannot be detected. Thus, there is a problem that a displacement between an actual sample surface and a detected image occurs, and the detection precision is impaired.
In addition, since the ideal irradiation position caused by the electron beam control signal does not coincide with the actual irradiation position of the electron beam, the electron beam is not actually irradiates a user-specified irradiation position. That is, the electron beam is irradiates a different position. Thus, there is a problem that an SEM image at the irradiation position that is not specified by the user is recognized as an SEM image at the user-specified irradiation position.
From the foregoing, it is an object of the present invention to provide a method of correction of a scanning electron microscope capable of performing correction with high precision by causing the user-specified irradiation position of the electron beam to coincide with the actual irradiation position of the electron beam.
In order to achieve the foregoing object, according to a first aspect of the present invention, there is provided a scanning electron microscope correction method comprising the steps of: setting a detection sample for producing light of an intensity corresponding to an electron density of an electron beam irradiating a surface of the detection sample; irradiating with the electron beam a predetermined position of the detection sample placed on a movable stage of the scanning electron microscope; detecting the intensity of the light produced from the detection sample; and performing correction relating to the scanning electron microscope on the basis of the intensity of the detected light.
In the scanning electron microscope correction method according to the first aspect of the present invention, correction relevant to the scanning electron microscope is performed by, for example, utilizing the fact that the density of the electron beam irradiated to the surface of the detection sample is a maximum and the detected light intensity is a maximum when the focal position of the electron beam irradiating the detection sample is precisely converged on the surface of the detection sample. For example, at least one of the movement amount of the focal position of the electron beam and the movement amount of the stage is corrected.
In this way, for example, the user-specified focal position of the electron beam can coincide with an actual focal position of the electron beam accurately, and the focal position of the electron beam can be corrected with high precision.
A focal position can be corrected by correcting, for example, the movement amount of the focal position so that the detected light intensity is always a maximum at each of the different positions on the detection sample on an inactive stage or the movement amount of the stage so that the detected light intensity is always a maximum in a state in which the movement amount of the focal position is constant (fixed). In addition, the focal position is moved in an optical axial direction so that the detected light intensity during electron beam scanning is always a maximum, whereby the focal position can be corrected.
According to a second aspect of the present invention, there is provided a scanning electron microscope correction method, wherein one of a movement control amount of a focal position of the electron beam in an optical axis direction and a movement control amount of the stage in the optical axis direction is corrected at a plurality of positions on the stage. In this manner, the focal position is corrected corresponding to an inclination of the stage. Thus, the displacement of the focal position due to such inclination of the stage is prevented, and an error caused by the inclination of the stage can be eliminated.
First, an electron beam irradiates a predetermined position, and the movement control amount of the focal position in the optical axis direction is corrected. Then, the stage is moved, and the irradiation position of the electron beam is changed to another position, whereby the electron beam irradiates another position. If the stage is inclined, the focal position is shifted from the surface of the detection sample, and the maximum optical intensity is not obtained.
In order to maximize the light intensity, there can be employed a method of adjusting the movement control amount of the focal position in the optical axis direction at each position after the stage has been moved; and a method of adjusting the inclination of the stage at each position after the stage has been moved.
According to the former method, i.e., the method of adjusting the movement control amount of the focal position in the optical axis direction at each position after the stage has been moved, even if the stage is moved while it is inclined, the focal position is always on the detection sample. Thus, even if the stage is inclined, the sample can be well observed.
According to the latter method, i.e., the method of adjusting the inclination of the stage at each position after the stage has been moved, if the stage is not inclined, the focal position of the electron beam at each position is not shifted from the detection sample. Thus, the movement control amount of the focal position in the optical axis direction after the stage has been moved is set to zero. If the stage is inclined, the focal position of the electron beam at each position is shifted from the detection sample. Thus, the movement control amount of the focal position in the optical axis direction after the stage has been moved is not set to zero, which corresponds to the stage inclination quantity.
Therefore, the inclination of the stage is adjusted so that the movement control amount of the focal position in the optical axis direction after the stage has been moved is set to zero, whereby the stage is accurately set so that the inclination of the stage is eliminated when the initially corrected position is defined as a reference, thus enabling correction with high precision.
According to a third aspect of the present invention, there is provided a scanning electron microscope correction method according to the first or second aspects thereof, wherein the movement control amount of the stage in the optical axis direction and the movement control amount of the focal position of the electron beam in the optical axis direction are corrected on the basis of a correlation between a predetermined movement amount of the stage in the optical axis direction, and a movement amount of the focal position of the electron beam in the optical axis direction when the focal position of the electron beam is moved in the optical axis direction so that the light intensity is a maximum after the stage, in a state in which the focal position of the electron beam is set on the detection sample, has been moved in the optical axis direction by the predetermined movement amount.
When the stage is moved in the optical axis direction after the focal position has been corrected, the focal position is shifted from the detection sample. This displacement quantity corresponds to the actual movement amount of the stage in the optical axis direction. In addition, the displacement quantity corresponds to a distance that correlates with the movement amount (or the adjustment amount) of the focal position of the electron beam when the detected light is at the maximum light intensity after the stage has moved in the optical-axis direction.
According to the third aspect of the present invention, the movement control amount of the stage in the optical axis direction and the movement control amount of the focal position of the electron beam in the optical axis direction are corrected based on a correlation between the movement amount of the focal position of the electron beam in the optical axis direction and the movement amount of the stage in the optical axis direction. In this manner, actual movement of the stage in the optical axis direction can accurately coincide with movement of the focal position of the electron beam in the optical axis direction, thus enabling correction with high precision. The actual movement amount of the stage in the optical axis direction may be measured by employing a position detecting apparatus such as interference gauge.
According to a fourth aspect of the present invention, there is provided a scanning electron microscope correction method according to any of the first to third aspects thereof, wherein a plurality of detection samples are disposed on the stage at known intervals, and a movement control amount of the stage with respect to an actual movement amount of the stage is corrected on the basis of the movement control amount of the stage from a position at which the electron beam irradiates a first detection sample which is one of the plurality of detection samples to a position at which the electron beam irradiates a second detection sample which is another of the plurality of detection samples, at the time the stage is moved linearly such that the electron beam irradiates the first detection sample and the second detection sample, and on the basis of a distance between the first detection sample and the second detection sample.
According to a ninth aspect of the present invention, there is provided a scanning electron microscope correction method for two-dimensionally scanning an electron beam with respect to a sample, thereby two-dimensionally detecting secondary electrons irradiated from (emitted from or by) the sample and reading a secondary electron image, said method employing a plurality of detection samples, each the detection samples generating light of a predetermined intensity corresponding to an electron density of the electron beam irradiated onto a surface of respective detection samples, wherein the plurality of detection samples are disposed on the stage at known intervals, and a stage movement control amount with respect to an actual stage movement amount is corrected on the basis of a stage movement control amount from a position at which an electron beam irradiates a first detection sample which is one of a plurality of detection samples to a position at which an electron beam irradiates a second detection sample which is another of the plurality of detection samples, at the time the stage is moved linearly such that the electron beam irradiates the first detection sample and the second detection sample, and on the basis of a distance between the first detection sample and the second detection sample.
A plurality of detection samples is disposed on a stage with predetermined intervals. The stage is linearly moved so that an electron beam having its focal position corrected passes over two samples between a first detection sample and a second detection sample. The detection sample is disposed on the stage with the known intervals. Thus, the movement control amount of the stage (stage movement control amount) required for actually linearly moving the electron beam from the first detection sample to the second detection sample corresponds to a known distance between the first detection sample and the second detection sample. Here, the stage movement control amount can be obtained based on light generation in the first detection sample and light generation in the second detection sample.
Therefore, a unit movement control amount of the stage with respect to an actual unit movement distance of the stage can be accurately detected based on the known distance and the stage movement control amount, and thus, the stage movement control amount with respect to the actual stage movement amount can be corrected with high precision.
According to a fifth aspect of the present invention, there is provided a scanning electron microscope correction method according to any one of the first to fourth aspects thereof, wherein the detection sample is formed in a rectangular parallelepiped shape with a width that is almost equal to or smaller than a diameter of the electron beam in a transverse direction of the detection sample, and when the detection sample is placed on the stage and the electron beam irradiates the detection sample and one of that the electron beam is scanned or that the stage is moved is performed, at least one of a scanning direction of the electron beam or a movement direction of the stage is corrected so that a light detection time becomes the longest.
According to a tenth aspect of the present invention, there is provided a scanning electron microscope correction method for two-dimensionally scanning an electron beam with respect to a sample, thereby two-dimensionally detecting secondary electrons irradiated from (emitted from) the sample and reading a secondary electron image, wherein a detection sample is formed in a rectangular parallelepiped shape with a width that is almost equal to or smaller than a diameter of the electron beam in a transverse direction of the detection sample, and when the detection sample is placed on the stage and the electron beam irradiates the detection sample and one of that the electron beam is scanned or that the stage is moved is performed, at least one of a scanning direction the electron beam or a movement direction of the stage is corrected so that a light detection time becomes the longest.
When a stage moves in a direction orthogonal to an optical axis or when an electron beam is scanned in a direction orthogonal to the optical axis, the irradiation position of the electron beam on the stage moves in a direction opposite to the stage movement direction or in a direction identical to the electron beam scanning direction. At this time, when the electron beam passes over the detection sample in a rectangular parallelepiped shape, light is irradiated from the surface of the detection sample in the rectangular parallelepiped. A time during which this light is irradiated corresponds to a distance on the detection sample surface in the rectangular parallelepiped on which the electron beam actually moves if the stage movement velocity or the electron beam scanning velocity is constant. Thus, a case where the detection time of light to be detected becomes the longest corresponds to a case in which the electron beam has actually moved along the longitudinal direction of the detection sample in the rectangular parallelepiped.
Therefore, the stage movement direction or electron beam scanning direction when the detection time of light to be detected becomes the longest corresponds to a direction in which the electron beam has actually moved along the longitudinal direction of the detection sample. At least one of the electron beam scanning direction and the stage movement direction is corrected so that the light detection time becomes the longest.
In this manner, the stage movement direction can coincide with the actual electron beam movement direction precisely, thus enabling correction with high precision.
According to a sixth aspect of the present invention, there is provided a scanning electron microscope according to the fifth aspect thereof, wherein, after the stage, in a state in which the movement direction of the electron beam on the detection sample is in a longitudinal direction of the detection sample, is rotated around an optical axis over a predetermined arbitrary angle, and the electron beam irradiates the detection sample and one of that the electron beam is scanned or that the stage is moved is performed, a stage rotation control amount for rotating the stage is corrected so that an angle of intersection, between the one of the scanning direction of the electron beam or the movement direction of the stage in which a light detection time becomes the longest and the one of the scanning direction of the electron beam or the movement direction of the stage in which a light detection time before the stage is rotated around the optical axis over the predetermined arbitrary angle becomes the longest, coincides with the predetermined arbitrary angle.
According to an eleventh aspect of the present invention, there is provided a scanning electron microscope correction method according to the tenth aspect thereof, wherein, after the stage, in a state in which the movement direction of the electron beam on the detection sample is in a longitudinal direction of the detection sample, is rotated around an optical axis over a predetermined arbitrary angle, and the electron beam irradiates the detection sample and one of that the electron beam is scanned or that the stage is moved is performed, a stage rotation control amount for rotating the stage is corrected so that an angle of intersection, between the one of the scanning direction of the electron beam or the movement direction of the stage in which a light detection time becomes the longest and the one of the scanning direction of the electron beam or the movement direction of the stage in which a light detection time before the stage is rotated around the optical axis over the predetermined arbitrary angle becomes the longest, coincides with the predetermined arbitrary angle.
After correction of the stage movement direction has been performed, when the stage is rotated by an predetermined arbitrary angle, thereby detecting a predetermined direction in which a detection time of light to be detected becomes the longest, the predetermined direction is in a direction in which the electron beam actually moves a long the longitudinal direction of the detection sample.
A crossing angle between the detected predetermined direction (longitudinal direction of the detection sample after the stage has been rotated by the predetermined arbitrary angle) and a direction (longitudinal direction of the detection sample) detected before the stage is rotated corresponds to a stage rotation quantity. Thus, the crossing angle ideally coincides with the predetermined arbitrary angle. If the stage rotation control amount is deviated from the actual stage rotation angle, the stage rotation control amount for rotating the stage is corrected so that the crossing angle coincides with the predetermined arbitrary angle. In this manner, the actual stage rotation control amount and the stage rotation angle corresponding to the rotation control amount can be accurately corrected.
According to a seventh aspect of the present invention, there is provided a scanning electron microscope correction method according to the fifth or sixth aspect thereof, wherein at least two detection samples in rectangular parallelepiped shapes are placed on the stage parallel to each other,
and when the electron beam irradiates one of the detection samples in rectangular parallelepiped shapes and one of that the electron beam is scanned or that the stage is moved is performed, at least one of the electron beam scanning amount and the stage movement amount is corrected with the one of the scanning direction of the electron beam or the movement direction of the stage, in which the light detection time becomes the longest being defined as a reference direction,
and when an electron beam irradiates another of the detection samples in a rectangular parallelepiped shape, and one of that the electron beam is scanned or that the stage is moved is performed, at least one of the electron beam scanning direction and the stage movement direction is corrected so that the one of the scanning direction of the electron beam or the movement direction of the stage when the light detection time becomes the longest coincides with the reference direction.
According to a twelfth aspect of the present invention, there is provided a scanning electron microscope correction method for two-dimensionally scanning an electron beam with respect to a sample, thereby two-dimensionally detecting secondary electrons irradiated from the sample and reading a secondary electron image, wherein at least two detection samples in rectangular parallelepiped shapes are placed on the stage parallel to each other, and when the electron beam irradiates one of the detection samples in rectangular parallelepiped shapes and one of that the electron beam is scanned or that the stage is moved is performed, at least one of the electron beam scanning amount and the stage movement amount is corrected with one of the scanning direction of the electron beam or the movement direction of the stage, in which the light detection time becomes the longest being defined as a reference direction, and when an electron beam irradiates another of the detection samples in a rectangular parallelepiped shape, and one of that the electron beam is scanned or that the stage is moved is performed, at least one of the electron beam scanning direction and the stage movement direction is corrected so that the one of the scanning direction of the electron beam or the movement direction of the stage when the light detection time becomes the longest coincides with the reference direction.
At least two detection samples in rectangular parallelepipeds shape are placed on a stage in parallel to each other. After correction in the stage movement direction has been performed by employing one of these detection samples in rectangular parallelepipeds, when there is detected a direction (one of the scanning direction of the electron beam or the movement direction of the stage) in which the detection time of light becomes the longest in another detection sample in a rectangular parallelepiped shape, the direction is in a direction in which the electron beam actually moves along the longitudinal direction of the other detection sample.
The detected direction ideally coincides with a longitudinal direction of the one detection sample in a rectangular parallelepiped when correction in the stage movement direction is performed by employing the one detection sample in a rectangular parallelepiped. However, if the parallelism of beam scanning is shifted or if an error occurs with the parallelism in the stage movement direction, such coincidence is not obtained.
Because of this, at least one of the electron beam scanning direction and the stage movement direction is corrected so that the detected direction coincides with the longitudinal direction of the one detection sample in a rectangular parallelepiped when correction in the stage movement direction is performed by employing the one detection sample in a rectangular parallelepiped. In this manner, the precision in parallelism of electron beam scanning can be corrected with high precision.
According to an eighth aspect of the present invention, there is provided in a scanning electron microscope correction method according to anyone of the first to seventh aspects thereof, wherein the detection sample is formed of a material that generates light of a single wavelength by being the electron beam irradiated, the detection sample being formed in a substantially rectangular parallelepiped shape with a width that is almost equal to or smaller than a diameter of the electron beam in a transverse direction of the detection sample, one side face of two side faces of the detection sample being a reflection surface for reflecting the light of the single wavelength, the light that transmits within the detection sample and reaches the other side surface is detected at the other side surface,
an actual irradiation position of the electron beam is detected based on the detected light intensity, and a scanning control amount of the electron beam is corrected so that a designated electron beam irradiation position coincides with the actual electron beam irradiation position.
According to a thirteenth aspect of the present invention, there is provided a scanning electron microscope correction method for two-dimensionally scanning an electron beam with respect to a sample, thereby two-dimensionally detecting secondary electrons irradiated from the sample and reading a secondary electron image, wherein a detection sample is formed of a material that generates light of a single wavelength by being the electron beam irradiated, the detection sample being formed in a substantially rectangular parallelepiped shape with a width that is almost equal to or smaller than a diameter of the electron beam in a transverse direction of the detection sample, one side face of two side faces of the detection sample being a reflection surface for reflecting the light of the single wavelength, the light that transmits within the detection sample and reaches the other side surface is detected at the other side surface, an actual irradiation position of the electron beam is detected based on the detected light intensity, and a scanning control amount of the electron beam is corrected so that a designated electron beam irradiation position coincides with the actual electron beam irradiation position.
Light is irradiated from the irradiation position of an electron beam on a detection sample, and the irradiated light propagates (transmits) the inside of the detection sample. According to the present invention, there is employed a columnar detection sample that comprises a reflection surface, for reflecting the light with its single wavelength, which is one of side surfaces opposing each other, wherein a part of the light generated from an electron beam irradiation position (referred to as a first light) propagates the inside of the detection sample, and reaches the reflection surface, and is reflected on the reflection surface to go to another side surface opposite to the reflection surface.
In addition, another part of the light generated from the electron beam irradiation position (referred to as a second light) goes to the side surface opposite to the reflection surface. Thus, the light going to the side surface opposite to the reflection surface from the electron beam irradiation position becomes interference light due to the first light and the second light.
Here, a relative phase angle of the first light is equal to that of the second light, the first and second lights are mutually emphasized, and an amplitude is a maximum. If the relative phase angles are 180 degrees, these light are offset, and the amplitude is minimal, i.e., 0. A factor of determining this relative phase difference lies in an optical path length (distance) D from a light generation point (i.e., light irradiation position) to the reflection surface. Assuming that the wavelength is xcex, when D=((xcex/2)xc3x97(2n)) (provided if xe2x80x98nxe2x80x99 denotes a natural number), the relative phase angles of the first and second lights is set to 0 degree, and the first and second lights are mutually emphasized. When D=((xcex/2)xc3x97(2n+1)), the relative phase angles of the first and second lights is set to 180 degrees, and the first and second lights are offset. By virtue of the above reason, the intensity of the light going to the side surface opposite to the reflection surface periodically changes gradually from the maximum intensity to the minimum intensity.
Therefore, the actual irradiation position (movement amount) of the electron beam can be accurately detected based on a relationship between an electron beam scanning distance and the detected light intensity.
In this manner, there can be detected the quantity of displacement between the actual irradiation position (movement amount) of the detected electron beam and an irradiation position of the electron beam caused by scanning, and the electron beam irradiation position caused by scanning can be corrected with high precision according the displacement quantity.